Handler programming issues

The Handler class is truly fundamental to the Android platform and is used widely throughout, but there are plenty of ways we can get ourselves into trouble if we aren't careful.

Leaking implicit references

The biggest worry when using Handler within an Activity is resource leakage which, just as with AsyncTask, is very easy to do. Here's one of our earlier examples:

final Runnable runnable = new Runnable(){
    public void run() {
        // … do some work
    }
};
handler.postDelayed(runnable, TimeUnit.SECONDS.toMillis(10));

By declaring an anonymous inner Runnable inside an activity, we have made an implicit reference to that containing Activity instance. We've then posted the Runnable to a handler and told it to execute in 10 seconds time.

If the Activity finishes before the 10 seconds are up, it cannot yet be garbage collected because the implicit reference in our Runnable means that the Activity is still reachable by live objects.

So, although it makes for a concise...